La Niña Springs:  Start Out Typically Colder-than-Normal

A moderate to strong La Niña continues to be located across the equatorial waters of the Pacific Ocean.  Water temperatures continue to average between 1oC and 2oC below normal from 160oE to 100oW.  The image below is compliments of the Climate Prediction Center and it shows how the equatorial waters have remained rather abnormally cold  across much of the equatorial Pacific from mid December through early March.

Pacific Water Temperatures from December 2007 through March 2008

This cold water greatly influences the location of the jet streams across the globe and this in turn affects the track of low pressure systems.  As was the case for much of the winter, the main storm track has remained just to our south and east so far this spring.  The image below from the Climate Prediction Center show the track of low pressure systems from December 11, 2007 through March 9, 2008. 

Low Pressure Tracks from December 11, 2007 through March 9, 2008

This storm track has resulted in several bouts of heavy snow across southern and eastern Wisconsin, eastern third of Iowa, northern Illinois, and Michigan.  In addition, these tracks keep cooler-than-normal temperatures in the Upper Mississippi River Valley.  Local statistical analysis show that there is a strong correlation for cooler-than-normal temperatures from January into April across the Upper Mississippi River Valley.  In the analysis of the February-March-April average temperature data in La Crosse, 10 out of 15 (66.7%) La Niñas since 1950 produced colder-than-normal temperatures during this 3-month time period.  The composite analysis below shows that similar results are observed from the western United States northeast into the northern Great Lakes.



Typically from mid to late April and into May, the signal for temperatures becomes very unclear.  This becomes very apparent when you look at the temperatures composite analysis for the fifteen La Niña's that have occurred during the April-May-June time frame since 1950.  Five La Niñas showed up in each of the terciles (above-normal, near-normal, and below-normal).  As a result, La Niña springs are usually delayed in their warm up across the Upper Mississippi River, but then there is significant differences in temperatures that cannot be explained necessarily by La Niña when looking at meteorological spring (March 1st through May 31st) as a whole. 

While there is a strong statistical signal for temperatures, there is no clear signal at all when it comes to spring precipitation.  This is mainly a result of large variations on how far northwest the primary storm track can move toward the Upper Mississippi River Valley.  In the precipitation composite analysis below, one can see that the primary storm track remains located from the Tennessee Valley northeast into the Great Lakes and New England (this is denoted by the above-normal precipitation). 


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